Understanding Multiple Inheritance Method Resolution Order in Python

This code demonstrates how Python's MRO determines which greet method is called in a diamond inheritance scenario, and prints the full MRO for class D.

Medium Python 3.9+ Aug 9, 2026 OOP & classes 12 views 0 copies

multiple inheritance mro inheritance oop

Python code

20 lines

Python 3.9+

class A:
    def greet(self):
        return "Hello from A"

class B(A):
    def greet(self):
        return "Hello from B"

class C(A):
    def greet(self):
        return "Hello from C"

class D(B, C):
    pass


if __name__ == "__main__":
    d = D()
    print(d.greet())
    print(D.__mro__)

Output

stdout

Hello from B
(<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <class 'object'>)

How it works

Python uses the C3 linearization algorithm to compute the Method Resolution Order (MRO) for classes with multiple inheritance. In this example, class D inherits from both B and C, creating a diamond pattern. The MRO ensures that each class appears exactly once and that the order is consistent with the inheritance dependencies, so B is prioritized over C because B appears first in D's base list. The __mro__ attribute shows the full lookup order, which is why d.greet() resolves to B's method before C's or A's. This deterministic ordering prevents ambiguity in method resolution.

Common mistakes

Assuming MRO follows the order of the base classes in the class definition plus a left-to-right depth-first search without accounting for C3 linearization.
Forgetting that the MRO includes all ancestor classes and 'object' at the end.
Expecting that the most specific base class always wins; in diamond inheritance, the order of bases can matter.

Variations

Use `super()` to cooperatively call methods up the MRO.
Inspect `D.mro()` (without underscores) to get the same list programmatically.

Real-world use cases

Designing mixin classes where method precedence must be predictable to avoid unexpected overrides.
Building class hierarchies that combine multiple behavior sources while maintaining consistent method resolution.
Debugging complex OOP codebases to trace which method is actually called in a diamond-style inheritance.

Understanding Multiple Inheritance Method Resolution Order in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

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How it works

Common mistakes

Variations

Real-world use cases

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